This invention relates to an apparatus for detecting an abnormal feed condition such as the failure of paper feed occurring in, for example, a printer or copying machine.
In recent years, various types of electronic printing devices are put to practical use, in which sheets are taken out of a feeder one by one; data is impressed on a sensitized drum by means of an optical fiber tube; the impressed data is electrostatically transcribed on the sheet; and a sheet on which data is visibly printed is drawn out through the outlet port of the printing device. With the above-mentioned type of printing device, data is printed on the sheets while they are traveling through the printing device one after another, after having been taken out of the feeder at the prescribed time interval. Unless, therefore, the sheets are transported exactly at the prescribed time interval, the position of data printed on a sheet tends to be displaced; and if a sheet is not yet set in the print position, data alone is sometimes supplied and consequently wasted. Such a defective feed condition is mainly caused by the idle running of a feed roller which is designed to frictionally send copy sheets one by one from the feeder to, for example, a conveyor belt.
With the conventional printer and copying machine, a point of time at which a sheet is delivered from the feeder is defined by, for example, a timer. Where a sheet is not drawn out within the prescribed length of time, then this event is regarded as abnormal, and the printing device is stopped. If, in this case, the printing device is stopped as soon as the above-mentioned abnormal condition is detected, then a sheet which is being carried through the printing device is left therein. This retained paper has to be manually removed by opening the printing device. In this case, data impressed on the retained paper often still remains unfixed and consequently is wasted. Where the printing device is again put into operation, the unfixed data has to be supplied again to the printing device, complicating the control process. Further, the above-mentioned manual removal of the retained paper tends to give rise to errors in the subsequent setting of the printing device, probably leading to the occurrence of a fresh case of the failure of proper paper feed. Therefore, it may be considered advisable to attempt to stop the printing device after all the sheets carried by a conveyor belt have been drawn out of the printing device, instead of at the moment when the abnormal feed condition is detected. However, such an attempt would make it necessary to provide a separate timer or detection means and intricate control means, eventually complicating the arrangement of the printing device with the resultant cost increase.